I have a multi layer (8) PWB with blind vias going from 1 to 7 that is experiencing occasional blistering (delam). It is only on perhaps 15% of the boards run. We vacuum baked (85C, not sure what pressure) a sample lot and still had a few exhibit the problem. The vendor, recommened just baking for a longer time at a higher temperature (125C, 2hrs) and said the vacuum will not do anything for the problem. This seems counter intuitive to me. Can anyone shed some light on this

A good board used soon after you receive it should not delam. If only a small percentage is doing that I urge you to do some more looking at the board quality while you do the baking. Do a search of the archives on baking boards, Dave F had some good comments a couple of years ago. Baking is not a good long term solution.

If your board is small enough that your board vendor can make several out of each panel, then check the boards that delam to see if they all came from the same area of the panel. You may need to have the board vendor mark the boards with the location in the panel when they do the date code.

Check on how the boards are stored in your stockroom. Are they kept sealed? or exposed to moisture? What is the relative humidity like in your stockroom? I'm located on the gulf coast of Florida, so it is hard to keep our internal humidity levels down. I have to make sure our boards are stored in bags, and in cabinets, so they have less chance of absorbing moisture. If you will have them on hand for several months you should consider the heavy barrier bags that are moisture proof.